In food container laminates, aluminum foils have traditionally been used as sealing layers to protect the foodstuff from deterioration caused by contact with oxygen and moisture. Nowadays, the aluminum foils are to an increasing extent being replaced by various polymer films, which have good barrier properties, but which are more easily degradable in nature than aluminum. Since no single polymer can provide the same resistance to gas and water penetration as aluminum, the polymer-based laminates typically comprise multilayered polymer structures. Often, these structures incorporate thermoplastic polymers, such as polyolefins, which are heat sealable and thus make it possible to manufacture continuous structures. The polyolefins themselves are known to have excellent moisture and water vapor resistance, but they suffer from poor gas barrier properties. Therefore, the multilayer polymer structures usually include a specific gas barrier material, such as ethylene/vinyl alcohol (EVOH). A conventional polyethylene-EVOH-based multi-layer product has the following structure:
PE/adhesive polymer/EVOH/adhesive polymer/PE).
It is known that rigid rod polymers, like thermotropic liquid crystalline polymers (LCP), have excellent barrier properties. However, the processing of them is difficult due to anisotropic behavior. Because the price of the liquid crystalline polymers is also rather high, LCP's have not been used as barrier materials in practical applications.
U.S. Pat. No. 5,084,352 describes a multilayered barrier film product, which includes a first polymer having low permeability to moisture and a heterogeneous polymer blend film containing a gas barrier polymer. The heterogeneous film comprises a barrier polymer, such as EVOH, together with a second polymer, which is distributed within the barrier polymer. The second polymer comprises a polyolefin which is functionalized, so as to allow the heterogeneous polymer film to adhere to the first polymer film having low permeability to moisture. Among the alternative barrier polymers, which may not necessarily be utilized in packaging applications, liquid crystalline polymers are mentioned. No examples of the use of LC polymers are given nor are any concentration ranges suggested.
EP Patent Application No. 0 503 063 discloses a composite film comprising a liquid-crystal polymer layer containing a thermotropic liquid crystal polymer and a thermoplastic polymer layer laminated on at least one surface of the liquid crystal polymer layer. The polymer layer is laminated on the liquid crystalline polymer through an adhesive layer. The use of such a layer can be avoided by functionalizing the thermoplastic in order to ensure proper attachment to the LCP film.
Structures similar to the ones mentioned above are also described in JP Published Patent Applications Nos. 2 220 821, 2 253 949 to 2 253 951, and 2 261 456.
The main problem associated with the known multilayer films resides in the fact that the adjacent layers are comprised of different kinds of polymers and, in order to get them attached to each other, either an adhesive tie layer has to be applied between the adjacent layers or, as described above, the thermoplastic polymers have to be functionalized to provide adhesive properties.
According to the prior art, good barrier properties are only obtained if the LCP forms a homogeneous film layer, which means that the LCP proportion of the whole multi-layer structure becomes rather high. This increases the costs of the multilayered films.
Additionally, the preparation of the multilayered structures often comprises several manufacturing stages and can therefore be costly and time consuming.